We are all too familiar with uncontrolled runoff in landscape irrigation systems. Low head drainage is not only common throughout residential and commercial landscapes, it is ubiquitous. When we consider that turf grass consumes more irrigated acreage and water than the top five agricultural crops (Diep, 2011)it becomes incumbent upon us to use every drop of irrigated water wisely and efficiently.

One of the tools we use to provide optimal irrigation water to landscapes, that is widely supported by public agencies, is the Weather-based Irrigation Controller (WBIC). A core feature of all robust WBICs is a 'run-soak' cycle that is automatically programmed to reduce runoff. WBICs take inputs of soil, slope, precipitation rate and crop coefficients to maximize efficiency overall and to mitigate against runoff, particularly in sloped conditions. For example, under extreme slope conditions, a WBIC may dictate a 'run' duration of two minutes and a 'soak' duration of ten minutes to be repeated five times to provide ten minutes of irrigation. This study discusses the effectiveness of the 'run-soak' cycle and shows that it is greatly enhanced if drain checks are installed on the low-head sprinklers . This research posits that there are two sources of wasted water when runoff is uncontrolled: the amount of water that drains from the system each time the zone is turned off and; the amount of water that is used to re-fill the irrigation line that is not applied to the crop. That is to say, when the WBIC determines the optimal amount of water required for each zone, the water that is used to fill the line does not reach the crop in the time allotted. These two water sources are termed "fill water" and "runoff".

Research Methodology

This research study examines the consequences of uncontrolled irrigation runoff in landscape irrigation under conditions when a 'run-soak' cycle has been determined by the WBIC as necessary to mitigate runoff.

This study simulates uncontrolled runoff during five run-soak cycles of 2 minutes of run time. The study examines two sources of wasted water: runoff from low head drainage and; the water that is used to fill up the irrigation lines after drainage of the line occurs. The research simulates the performance of: PVC lines 1",2", 3", 4",6", 8", 10" and 12"; water velocities 5, 7 and 10 feet/second and; a fill-time of each line of 30 seconds. Rates of flow were calculated in Gallons per Minute and water capacity was calculated in Gallons per 100' (Diameter Velocity and Flow Rate)

Results

The following tables illustrate the data derived for the simulation:

Table 1 Flow rates at different velocities and Gallons per 100'

1"

2"

3"

4"

6"

8"

10"

12"

GPM at 5 ft/sec

12

48

110

196

440

783

1224

1762

GPM at 7 ft/sec

17

68

154

274

616

1090

1714

2467

GPM at 10 ft/sec

24

97

220

392

881

1567

2448

3525

1"

2"

3"

4"

6"

8"'

10"

12"

Gallons per 100'

4.5

17

38

66

150

260

410

588

http://www.1728.org/flowrate.htm

The flow rates for fill-time is calculated by dividing the Gallons per Minutes by one-half (30 seconds) and then multiplying the quotient by five to achieve fill-time for five cycles.

Table 2 Flow rate for fill-time and Flow rate for five cycles

Flow Rate in GPM for 30 second fill

1"

2"

3"

4"

6"

8"

10"

12"

GPM/2 at 5 ft/sec

6

24

55

98

220

391.5

612

881

GPM/2 at 7 ft/sec

8.5

34

77

137

308

545

857

1233.5

GPM/2 at 10 ft/sec

12

48.5

110

196

440.5

783.5

1224

1762.5

Flow Rate in GPM for 5 cycles

1"

2"

3"

4"

6"

8"

10"

12"

GPM/2*5 at 5 ft/sec

30

120

275

490

1100

1957.5

3060

4405

GPM/2*5 at 7 ft/sec

42.5

170

385

685

1540

2725

4285

6167.5

GPM/2*5 at 10 ft/sec

60

242.5

550

980

2202.5

3917.5

6120

8812.5

Page Break

The table below summarizes and illustrates the runoff water for five cycles, a 30 second fill, and a 30 second fill time five at different pipe sizes and different velocities:

Table 3 Runoff for five cycles, fill for one and fill for five cycles

30 Second fill

30 Second fill*5

Run Off for 5 cycles

5 ft/sec

7 ft/sec

10 ft/sec

5 ft/sec

7 ft/sec

10 ft/sec

1"

45

6

8.5

12

30

42.5

60

2'

170

24

34

48

120

170

240

3'

380

55

77

110

275

385

550

4'

660

98

137

196

490

685

980

6"

1500

220

308

440

1100

1540

2200

8"

2600

391

545

783

1955

2725

3915

10"

4100

612

857

1224

3060

4285

6120

12"

5880

881

1233

1762

4405

6165

8810

The aggregate of water wasted and the percentage of water wasted when compared to the theoretical application of ten minutes of water is represented below

Table 4 Total water wasted in quantity and percent

1"

2"

3"

4"

6"

8"

10"

12"

Gallons lost at 5 ft/sec

75

290

655

1150

2600

4555

7160

10285

Gallons lost at 7 ft/sec

87.5

340

765

1345

3040

5325

8385

12045

Gallons lost at 10 ft/sec

105

410

1360

1640

3700

6515

10220

14690

1"

2"

3"

4"

6"

8"

10"

12"

Gallons lost at 5 ft/sec

63%

60%

59%

59%

59%

58%

58%

58%

Gallons lost at 7 ft/sec

51%

50%

66%

49%

49%

49%

49%

49%

Gallons lost at 10 ft/sec

44%

42%

61%

42%

42%

42%

42%

42%

The following table is a summary illustration of the percent of water lost, or wasted water, derived from the data in this study. The table indicates that for all velocities, at all pipe sizes, the percent of water wasted is between 42 and 62% of total applied water when five 2minute cycles are programmed.

Table 5 Percent of total water loss

Discussion

WBICs are estimated to save between 15-30% of water applied when compared to non-WBICs (U.S.EPA, 2011).This study provides evidence that the potential for more efficient use of applied water is to control runoff. Controlling runoff, according to the evidence herein presented, saves between 42 and 63% of water in slope conditions when a run-soak cycle is applied. In order to control runoff, sprinklers that have built-in drain checks can help to mitigate the amount of wasted water but these sprinkler options are limited to the amount of head (pressure) that they can maintain. There are discrete drain checks on the market that can be integrated into the irrigation system to mitigate runoff under conditions of greater head (pressure).

Policy Implications

The implications for controlling runoff are greater than water conservation. Frequently, liquid fertilizers, largely composed primarily of nitrogen, are applied through the irrigation systems and runoff into the groundwater and ocean. In addition, when irrigation systems are pressurized by pumping systems the efficiency of these pumps is reduced and the cost of pumping is increased when significant amounts of water are wasted. Finally, investments in subsidizing the costs of other conservation tools while not recognizing the effectiveness of tools to mitigate runoff create overall inefficiencies in rebate programs.

Conclusion

The evidence of the simulation study illustrates that run-soak cycles are only effective to the extent that runoff is mitigated by drain checks on the lowest heads. The effects of controlling runoff are positive and significant and should be recognized as a critically important tool for water conservation and supported by public policy agents and institutions.

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